402 



THE CIRCULATION. 



veins which finally end in the vena cava b, and enters the 

 right auricle c. From this it is pumped* into the right ven- 

 tricle d, which, contracting in its turn, forces it on into the pul- 

 monary artery e, spreading out upon the lining membrane of the 



Fig. 691.— Theoretical Plan of the Circulatory System. 



H*D, C, E, the canal for red blood; E, B, A, G, canal for 

 blue blood. The arrows indicate the course of the blood. The 

 two canals are represented in their, middle portion; A, B, C, D, 

 as isolated; but in nature they are enveloped at' this point in a 

 common sac that concurs to form the heart. 



lungs, to form the capillaries of that organ at 

 /, from which it is returned to the left auricle 

 g through the pulmonary veins, from the left 

 auricle it is driven on through the left ven- 

 tricle ; and this, by its powerful contraction, 

 forces the blood through the aorta *', and the 

 arteries of the whole body to the capillaries a, 

 from which the description commenced. 



While the venous blood is on its way to the 

 heart, when near it, it is met by the tho- 

 racic duct (see k, y, Fig. 694) , which conveys 

 into this returning blood the nutritive prop- 

 erty of the food extracted from it by the 

 digestive organs. With this new supply of nutritious matter, the 

 blood goes to the heart and lungs to be oxygenized by contact 

 with the air, and thus be continued through the heart and/arteries 

 as before explained. The system takes up its material for its wear 

 and tear through the capillaries or hair-like tubes, which are inter- 

 posed between the two great divisions of the vascular system, ar- 

 teries and veins. These little tubes are so small that they are 

 from io'oo to ^oo of an inch in diameter. The smaller are found 

 the retina qf the eye and brain, the larger in the liver and 



Fig. 692.— Plan of the 

 . Circulation. 



in 



